Why do some items need 2 batteries?

Generally, it needs twice the voltage supplied by one battery. For example, a 2-cell flashlight using 2 of AA or C or D batteries is combining the voltage of 1.5 volts supplied by each cell to produce a total (average) of 3 volts to power a 3v bulb. The above example assumes the batteries are connected in series, or in line. If two batteries are connected in parallel (that is, head-to-head and tail-to-tail), the combination produces the same voltage as a single battery but can power a load for twice the time as would a single cell.